To simplify the following discussion, the present invention will be discussed in terms of displays utilized in head mounted computer displays; however, it will be apparent to those skilled in the art from the following discussion that the present invention may be applied to other types of displays. Head-mounted computer displays may be viewed as "eye glasses" that are worn by the user to view images created by a computer. The image seen by each eye is generated on a display screen having a two dimensional array of pixels.
In one type of display, each pixel is a small mirror that is covered by a "shutter" that is controlled by the voltage of the mirror. The shutter is constructed from a layer of liquid crystal on the mirrors. The voltage controls the state of the liquid crystal on top of the pixel so as to modulate the reflected light. A light source illuminates the pixels and the modulated reflected light from the pixels is imaged into the eye of the viewer. The imaging optics typically consist of lenses which magnify the pixels and form a virtual image. The light source is typically constructed from 3 LEDs that emit different colors.
For this type of display to function properly, the intensity of light reflected by each micro-mirror must be independent of the pixels location in the display. In addition, each pixel must appear to be an independent light source. In prior art systems, these constraints are met by converting the three point light sources into a diffuse light beam which strikes the display at right angles to the plane of the mirrors. The light source utilizes a condenser lens to collimate or slightly diverge the light to match the diffuser or telecentricity of the imaging optic and an array of micro-lenses in the collimated light beam to provide the required diffusion. Since the light source must be outside the field of view of the user so as not to block the image generated by the display, a half silvered mirror is used to illuminate the display while allowing light reflected by the display to reach the eye of the viewer.
This prior art solution to the illumination problem has several problems. First, the distance between the first imaging optic and the display must be at least as great as the shortest dimension of the display to provide room for the half-silvered mirror. Second, the illuminator requires a condenser lens and diffuser which must be at least as large as the display. These constraints lead to a bulky display. Both the size and the weight of this type of display are objectionable.
To collimate the light source, all of the LEDs must be very close to the focal point of the collimating lens and limited in size so as to simulate a single point source and properly mix the colors of the LEDs. This constraint limits the size of the LEDs, and hence, the maximum intensity of light from the display. In addition, the half-silvered mirror decreases the brightness of the display, since only one fourth of the light in the collimated beam actually reaches the viewer's eye.
Broadly, it is the object of the present invention to provide an improved illumination system for a reflective display.
It is a further object of the present invention to provide a display system that does not require the use of a half-silvered mirror to illuminate the pixels.
It is a still further object of the present invention to provide a display system that can utilize spatially separate color LEDs or other non-point light sources to provide increased image intensity, while still providing good color mixing.
It is yet another object of the present invention to provide a display system that does not require the use of a condenser lens nor a diffuser between the light source and the display.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.